My 996 Carrera Cabrio, 2001 model (6 speed manual, no cruise control) has a peculiar problem: When activating the brake switch (when braking) the engine stalls and dies, but when taking out the switch everything is fine. My Durametric comes up with various codes, such as P1219 (256), P1578 (428), P0121 (117), P1121 (430) and P1122 (431) when it happens, but all disappear once I disconnect the brake switch - but of course, the brake lights won't work without it. And yes, I have tried putting in a new switch - same result, even when jumping the plug without the switch. As part of fault finding I have traced all the wires to the 4 brake lights as well as to the ECU - all are OK with no shorts. Any suggestions?

 

What? I don't even know how to respond to this...and probably should not. You stomp on your brakes and your engine dies? Whatever correlation is between your brakes and your engine dies...I have not the slightest clue or idea. I cannot even imagine a situation of "I brake and my engine stalls and dies". You have got one serious problem.

 

I know u have checked for shorts but this really sounds like an electrical issue. I would pull the tail lights and unplug one bulb at a time to eliminate the assemblies. Then i would probably start jumpering sections of the harness. Its gonna be a big pita to find this one. Good luck.

 

https://www.renntech.org/topic/29701-p1219-p1577-p1578/

 

https://rennlist.com/forums/996-foru...ntiometer.html

 

I've seen this in variations a few times, it almost always traces back to:
1.bad ground.
2. wire damage
3. once it was a bad bulb in a rear socket back feeding though the tail light / headlight circuit back to the ecu. (BMW)
4. Once it was the rain sensing module fort he wipers.(BMW)
5. The Antilock brake system is a great source of malfunction as well. (All german cars,,, LOL)

Some part of the system gets isolated and you start feeding power basically backwards though other components.
Getting the car in the air and end to end inspecting everything you can get at, under carpets, look for ground straps that
are broken or come undone, abraded wires,

The way I've found it is to use a volt/ohm meter, attach a extension wire to the neg of
the battery and probe every bit of the car you can reach to see if there is current flow
from a component that should be grounded, then probe in ohm mode everything should
have very close to 0 ohms resistance back to battery neg. Basically all the chassis should
be 1 potential. Then you do it with power on on the car. then you do it the other direction
1 lead on hot and probe everything again, it should be 0 volts to any hot point and full
battery voltage to any grounded device. (Assuming switch positions are all off)

The other test is to hold the brake on while you pull and replace one fuse at a time,, you have to understand
what each fuse is running, your trying to find a circuit that could share a hot source and one side has a bad ground..

 

Thanks a lot for very sound advice - I'm anyway locked up with the Covid-19 threat so this will give me something worthwhile to do. Will report back

 

Thanks - I tried that with no result, all seems fine. There is one wire going to the ECU, but when I disconnect, the engine starts rough and dies the same way. Could be an ECU problem, but a little expensive just to change the ECU for fault finding.

 

Electrical gremlins are a special seat in hell sometimes..

 

Thanks, PDXMOTORHEAD - I have run through every wire up and down twice and everything checks out. Since all wires, connections, ground points and brake switch are OK I am afraid all there is left to do is send the ECU for testing - two wires are going there from the brake switch (both solid connection) so I assume the ECU is somehow playing up. Any thoughts?

 

Thanks - I tried all of your suggestions (twice) and all seem OK. Right now I'm so frustrated that I have even considered running separate wiring for the stoplights (only joking). Seems the only source left is the ECU, so I'll send it for testing, although this Covid-19 is really making any shipping difficult, so I'll have to wait patiently till the Indian lock-down is eased in a couple of weeks.

 

u can send the ecu to ecu doctors in florida for testing. Also i wonder if there is something wrong with the multifunction switch or even the ignition switch causing something to ground out. The power flow threw the column, ignition and headlight switches on these cars cause all kinds of strange issues. Personally i find when it seems like everything is pointing at the ecu it usually is not the problem .

 

Well, I'd like to avoid sending my ECU out so since I have ordered a new ignition switch anyway, I'll wait for that to arrive and give it one last try. I changed the headlight switch last week; little pieces of broken porcelain kept coming out of the old one.